This application claims priority to an application entitled xe2x80x9cDistortion Compensating Device and Method in a Multi-code Mobile Communication Systemxe2x80x9d filed in the Korean Industrial Property Office on Apr. 28, 1999 and assigned Serial No. 99-15222, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates generally to a multi-code mobile communication system, and in particular, to a device and method for compensating for signal distortion produced during multi-code transmission in a CDMA (Code Division Multiple Access) mobile communication system.
2. Description of the Related Art
The most challenging issue in the rapidly developing mobile communication industry is efficient use of limited radio frequency bandwidth. High speed transmission using multi-codes is one of the methods of efficiently providing radio multi-media service without increasing bandwidth. Multi-code transmission is the process of converting high rate data to a plurality of parallel low rate data streams, assigning an orthogonal code, and, thus, a code channel, to each parallel code channel which transmits each of the converted parallel low rate data streams, and combining the orthogonally spread code channels for transmission.
FIG. 1 is a block diagram of a typical transmitter in a multi-code using mobile communication system. The transmitter transmits data using four orthogonal codes. A transmitter 10 provides a signal to a specific receiver, or subscriber, and is one of a plurality of transmitters that are identical in structure.
Referring to FIG. 1, the transmitter 10 is comprised of a serial-to-parallel converter (SPC) 101, five multipliers 102 to 105 and 107, an adder 106, and an amplifier 108. The SPC 101 converts input serial user bit stream b1(t) into four parallel symbol bit streams b11(t) to b14(t). The multiplier 102 orthogonally spreads symbol bit stream b11(t) with orthogonal code W11(t) by multiplication. The multiplier 103 orthogonally spreads symbol bit stream b12(t) with orthogonal code W12(t) by multiplication. The multiplier 104 orthogonally spread symbol bit stream b13(t) with orthogonal code W13(t) by multiplication. The multiplier 105 orthogonally spreads symbol bit stream b14(t) with orthogonal code W14(t) by multiplication. The adder 106 sums the spread symbols bits received from the multipliers 102 to 105 on a bit basis. The multiplier 107 PN-spreads the output of the adder 106 with a first PN code PN1(t) by multiplication. The amplifier 108 amplifies the PN-spread signal received from the multiplier 107, for transmission.
As described above, the conventional transmitter converts a signal to a specific subscriber into a parallel signal, spreads each parallel signal with a different orthogonal code, sums the spread signals, spreads the sum with a predetermined PN code, and amplifies the PN-spread signal. The spreading of different orthogonal codes is considered multi-code communication.
Signals output from transmitters #1 to #k can be considered summed in the transmission channel as indicated by reference numeral 11. During propagation in the transmission channel, which may be the atmosphere, the signal is combined with AWGN (Additive White Gaussian Noise).
FIG. 2 is a block diagram of a conventional receiver corresponding to the conventional transmitter using multi-codes. While a plurality of receivers #1 to #k are shown, a receiver 20 for a specific subscriber will be described as representative of them all since they are identical in structure.
Referring to FIG. 2, the receiver 20 is comprised of multipliers 201 to 209, accumulators 210 to 213, deciders 214 to 217, and a parallel-to-serial converter (PSC) 218. The multiplier 201 PN-despreads an input signal r(t) with its own PN code PN1(t) by multiplication. The multipliers 202 to 205 multiply the PN-despread signal by a conjugate signal xcex21ejxcfx86 for channel compensation. The multipliers 206 to 209 orthogonally demodulate the channel-compensated signals received from the multipliers 202 to 205 with corresponding orthogonal codes W11(t) to W14(t) by multiplication. The orthogonal codes are the same as those used in the transmitter 10 of FIG. 1. The accumulators 210 to 213 accumulate the despread signals received from the multipliers 206 to 209 in symbol units. The deciders 214 to 217 decide symbol bits from the outputs of the accumulators 210 to 213 and output the decision results. The PSC 218 converts the symbols received in parallel according to the decision results of the decider 214 to 217 to serial symbol bits and outputs a symbol bit sequence b1(t). The PSC 218 is the counterpart of the SPC 101 of FIG. 1.
The receiver 20 PN-despreads an input signal, separates the PN-despread signal. into as many signals as the number of the multi-codes used, in the above example, four signals, multiplies each of the signals with an orthogonal code for orthogonal demodulation, accumulates the orthogonally demodulated signals in symbol units, and then decides symbol bits. Then, the decided signals are converted to a serial signal, which are the symbol bits obtained in the receiver 20.
A signal spread by multi-codes passes through a transmitter amplifier with an increased peak-to-average power ratio, relative to a signal spread by a single code. In general, the high power amplifier (HPA) used as the transmitter amplifier in a multi-code system shows a non-linear characteristic. Therefore, the nature of the HPA causes signal distortion when its saturation point is set too high. That""s why the saturation point of the transmitter amplifier is decreased to reduce signal distortion in a transmitter. However, a decrease in the saturation point leads to a corresponding decrease of amplification efficiency. In view of the importance of power consumption to the performance of a mobile station, the above transmitter amplifier is not suitable for a mobile station since it increases power consumption.
Accordingly, there is a need for a receiver that compensates for the signal distortion generated when a signal passes through a transmitter amplifier with a high saturation point in order to prevent deterioration of BER (Bit Error Rate) performance. The high probability of errors in the distorted signal during propagation in the atmosphere reinforces the need for an error compensating device in the receiver.
It is therefore, an object of the present invention to provide a device and method for effectively compensating for the distortion of an input signal caused by an HPA in a multi-code mobile communication system.
It is another object of the present invention to provide a device and method for correcting errors of a parallel code channel by checking the parity of the code channel with parity data received on a redundancy channel.
Briefly, these and other objects are achieved by providing a receiving device in a multi-code mobile transmission system. In the novel multi-code mobile communication system, user data is transmitted on a plurality of parallel data channels and parity data generated from the user data is transmitted on a redundancy channel. An energy calculator of the receiving device calculates the energy of each symbol output by deciders and indicates the symbol with the smallest energy. A parity checker determines whether errors have been generated by checking the parity of symbol data, and a sign inverter inverts the sign of the symbol with the smallest energy (as indicated by the energy calculator), if it is determined that errors have been generated. Thus, errors are corrected.